I've come across conflicting information about Medusa's sisters and whether or not they could turn people to stone like Medusa can.

A lot of places online say that all three can; but some other places I've seen claim that only Medusa can, and I seem to remember this from reading into Greek mythology as a kid, so I'm skeptical of the more popular opinion that they all can, and was hoping for something more authoritative to state one way or the other.

[The Gorgons] were Stheno, Euryalë, [and] Medousa. […] They turned anyone who saw them to stone.

(Translation mine; I can't seem to find an edition with line numbers to cite.)

I can't find any other sources that mention Sthen(n)o and Euryalë being able to petrify, or not. As Lauren mentioned, Homer and many other authors mentions only a single Gorgon (sometimes "Gorgo"), whose head was on Athena's shield; the sisters don't show up much outside the Perseus myth.

Apart from the petrification, Hesiod similarly mentions that all three were monsters; Pseudo-Apollonius says all three were identical in appearance; Ovid says that all three were originally beautiful, but Medousa had her hair turned into snakes. Medousa's defining characteristic seems to have been that she was mortal, while her sisters weren't, which is why Perseus was able to kill her:

[There lived] Sthenno, and Euryalë, and Medousa, who suffered such misery. Only she [Medousa] was mortal, while the others are undying and unaging, the two of them; she [Medousa] is the only one blue-haired [Poseidon] wandered with amid the soft meadows and springtime flowers.

(Hesiod Theogony 276-9, translation mine.)

This is the only mention I can find of multiple Gorgons in the Theogony, with no mention of petrification. Many authors specify that Medousa could petrify people, but I can't find any specific mention that her sisters couldn't.

Medusa was the only mortal of the three sisters, and the only beautiful one. Poseidon raped her in one of Athena's temples. The other gods demanded that Medusa be punished for "defiling" the temple (never mind punishing Poseidon for raping her).

Athena changed her into a hideous creature with snakes for hair — interpretations vary on whether this was punishment for defiling the temple or a way to protect her from further abuse. Medusa was now so hideous that anyone who looked at her turned to stone.

Myths about the Gorgons range. Earlier poets (Aeschylus) called all three monsters, or said there was only one Gorgo altogether (Homer and Euripides). Later stories have the details about Medusa's rape and transformation (such as Ovid's Metamorphoses.) Hesiod in Theogony seems to split the difference, saying all three were monsters but that only Medusa had the petrification powers.

I'm not sure your source is conclusive: from that same page, I can find evidence to the contrary (emphasis mine): "All who looked at them were turned to stone." and "Have you seen the eye of Sthenno which turns all to stone", although all other mentioning of being turned to stone is specific to Medusa...
– NathanSJun 28 at 12:01

@NathanS the sources themselves are not conclusive because the myth(s) changed. The original story was "One monster named Gorgo, no N, singular." Then it was "All three Gorgons are monsters, but nobody turned to stone." Then it became "two monsters and one pretty mortal who was later cursed to turn people to stone."
– Lauren IpsumJun 28 at 16:58

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I can't seem to find anything about petrification in the Theogony, just that Medūsa was the only mortal one.
– DraconisJun 29 at 3:10

It's worth point out that Ovid had something of an anti-authoritarian grudge thanks to his treatment by the Roman Emperor, and his take on the Gorgon myth may be a reflection of that.
– nick012000Jun 29 at 11:43

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I suggested an edit to the "rape" part, since the provided sources do not mention anything like that and thus do not support that interpretation.
– plocksJun 29 at 13:47